The properties of concrete depend on the mixture (quantities) and quality of its components. Because cement, typically portland cement, is the active component of concrete and usually has the greatest unit cost, its selection and proper use are important in obtaining the balance of properties meeting user needs.
Type I and type I/II portland cements are the most popular cements used. However, some applications require other types for meeting user requirements. The need for high early strength cements in pavement repairs and the use of blended cements with aggregates susceptible to alkali-aggregate reactions are examples of such applications. The choice involves the correct knowledge of the relationship between cement and performance and, in particular, between type of cement and durability of the resultant concrete.
ASTM C 150 defines portland cement as “hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverizing clinkers consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an interground addition.” Clinkers are nodules (approximate diameters, 5-25 mm [0.2-1.0 inch]) of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature. The low cost and widespread availability of limestone, shales, and other naturally occurring materials make portland cement one of the least expensive building materials widely used over the last century.
Different types of portland cement meet physical and chemical requirements for specific purposes, such as durability and high early strength. Eight types of cement are covered in ASTM C 150 and AASHTO M 85. These types and their uses are listed in Table 1.
TABLE 1Portland Cement Types and UsesCEMENT TYPEUSEI1General purpose cement, when there are noextenuating conditionsII2Aids in providing moderate resistance to sulfateattackIIIWhen high-early strength is requiredIV3When a low heat of hydration is desired (in massivestructures)V4When high sulfate resistance is requiredIA4A type I cement containing an integral air-entraining agentIIA4A type II cement containing an integral air-entraining agentIIIA4A type III cement containing an integral air-entraining agent1Cements that simultaneously meet requirements of Type I and Type II, commonly referred to as Type I/II, are also widely available.2Type II low alkali (total alkali as Na2O <0.6%) is often specified in regions where aggregates susceptible to alkali-silica reactivity are employed.3Type IV cements are available only on special request.4These cements are in limited production and not widely available.
More than 92% of portland cement produced in the United States is Type I and II (or Type I/II). Type III accounts for about 3.5% of cement production. U.S. Department of the Interior, Bureau of Mines, Cement Mineral Yearbook, Washington, D.C., 1989. Type IV cement is available only on special request, and Type V may also be difficult to obtain, accounting for less than 0.5% of production.
If a given type of cement is not available, comparable results can frequently be obtained by using modifications of available types. High early strength concrete, for example, may be made by using a higher content of Type I when Type III cement is not available or by using admixtures such as chemical accelerators or high-range water reducers (HRWR). National Material Advisory Board, Concrete Durability: A Multi-Billion Dollar Opportunity, NMAB-437, Washington, D.C., National Academy Press, 1987.
The composition of portland cements is what distinguishes one type of cement from another. ASTM C 150 and AASHTO M 85 give the standard chemical requirements for each type. The phase compositions in portland cement are denoted by ASTM as tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). Note that these compositions occur at a phase equilibrium of all components in the mix and do not reflect effects of bum temperatures, quenching, oxygen availability, and other real-world kiln conditions. The actual components are often complex chemical crystalline and amorphous structures, denoted by cement chemists as “alite” (C3S), “belite” (C2S), and various forms of aluminates. The behavior of each type of cement depends on the proportions of these components.
Studies have shown that early hydration of cement is principally controlled by the amount and activity of C3A, balanced by the amount and type of sulfate interground with the cement. C3A hydrates rapidly and will influence early bonding characteristics. Abnormal hydration of C3A and poor control of this hydration by sulfate can lead to such problems as flash set, false set, slump loss, and cement-admixture incompatibility. Previte, R., Concrete Slump Loss, ACI Journal Proceedings 74 (8):361-67, 1977; Whiting, D., Permeability of Selected Concretes, ACI special publication, Permeability of Concrete SP-108: 195-222, 1988; Meyer, L. M. and W. F. Perenchio, Theory of Concrete Slump Loss as Related to Use of Chemical Admixtures, Concrete International, Design and Construction 1 (1):36-43, 1979.
Development of the internal structure of hydrated cement, referred to as the microstructure, occurs after the concrete has set and continues for months, and often years, after placement. The microstructure of the cement as it hydrates determines the mechanical behavior and durability of the resultant concrete. In terms of cement composition, C3S and C2S have the primary influence on long-term development of structure, although aluminates may contribute to formation of compounds such as ettringite (sulfoaluminate hydrate), which may cause expansive disruption of concrete. Cements high in C3S, in particular those that are finely ground to less than about 40 microns, hydrate rapidly and lead to high early strength. However, hydration products adversely affect hydration as the concrete ages, in some cases leading to an ultimate strength lower than desired. Cements high in C2S hydrate slowly, leading to a dense ultimate structure and a high long-term strength. The relative ratio of C3S to C2S, and the overall fineness of cements, has been steadily increasing over the past few decades. This ability to achieve desired strengths at a higher workability, and hence a higher water content (w/c), may account for many durability problems, as it is now established that higher w/c invariably leads to higher permeability in the concrete. Ruettgers, A., E. N. Vidal, and S. P. Wing, An Investigation of the Permeability of Mass Concrete with Particular Reference to Boulder Dam, ACI Journal Proceedings 31:382-416, 1935; Whiting, 1988.
One of the major aspects of cement chemistry that concerns users is the influence of chemical admixtures on portland cement. Since the early 1960's most states have permitted or required the use of water-reducing and other admixtures in highway pavements and structures. Mielenz, R., History of Chemical Admixtures for Concrete, Concrete International: Design and Construction 6 (4):40-54 (April), 1984. A wide variety of chemical admixtures have been introduced to the concrete industry recently, and engineers are increasingly concerned about the effects of these admixtures on performance.
Considerable research dealing with admixtures has been conducted. Air-entraining agents are widely used in the highway industry in North America, where concrete is subjected to repeated freeze-thaw cycles. Air-entraining agents have no appreciable effect on the rate of hydration of cement or on the chemical composition of hydration products. Ramachandran, V. S. and R. F. Feldman, Cement Science, In Concrete Admixtures Handbook: Properties, Science, and Technology, ed. V. Ramachandran, 1-54, Park Ridge, N.J., Noyes Publications, 1984. However, an increase in cement fineness or a decrease in cement alkali content generally increases the amount of an admixture required for a given air content. ACI Comm. 225R 1985. Water reducers or retarders influence cement compounds and their hydration. Lignosulfonate based admixtures affect the hydration of C3A, which controls the setting and early hydration of cement. C3S and C4AF hydration is also influenced by water reducers. Ramachandran and Feldman (1984).
ASTM C 150 and AASHTO M 85 specify certain physical requirements for each type of cement. These properties include 1) fineness, 2) soundness, 3) consistency, 4) setting time, 5) compressive strength, 6) heat of hydration, 7) specific gravity, and 8) loss of ignition. Each one of these properties has an influence on performance. The fineness of the cement, for example, affects the rate of hydration. Greater fineness increases the surface available for hydration, causing greater early strength and more rapid generation of heat, e.g., the fineness of Type III is higher than that of Type I cement. U.S. Department of Transportation, Federal Highway Administration, Portland Cement Concrete Materials Manual, Report no. FHWA-Ed-89-006 (August). Washington, D.C., FHWA, 1990.
ASTM C 150 and AASHTO M 85 specifications are similar except with regard to fineness of cement. AASHTO M 85 requires coarser cement that results in high ultimate strengths and low early strength gain. The Wagner Turbidimeter and the Blaine air permeability test for measuring cement fineness are both required by the American Society for Testing Materials (ASTM) and the American Association for State Highway Transportation Officials (AASHTO). Average Blaine fineness of modern cement ranges from 3,000 to 5,000 cm2/g.
Soundness, i.e., the ability of hardened cement-based paste to retain its volume after setting, is characterized by measuring the expansion of mortar bars in an autoclave. ASTM C 191, AASHTO T 130. The compressive strength of 50-mm (2-inch) mortar cubes after seven days, as measured by ASTM C 109, should be greater than 19.3 MPa (2,800 psi) for Type I cement. Other physical properties included in both ASTM C 150 and AASHTO M 95 are specific gravity and false set. False set is a significant loss of plasticity shortly after mixing due to the formation of gypsum or the formation of ettringite. In many cases, workability may be restored by remixing the concrete before casting.
Effects of the type of cement on the most important concrete properties are presented in Table 2. Cement composition and fineness control concrete properties. Fineness of cement affects the placeability, workability, and water content of a mixture much like the amount of cement used does.
TABLE 2Effects of Cement Property on Concrete CharacteristicsCHARACTERISTICCEMENT PROPERTYPlaceabilityamount, fineness, setting characteristicsStrengthcomposition (C3S, C2S and C3A), loss onignition, finenessDrying ShrinkageSO3content, compositionPermeabilitycomposition, finenessResistance to SulfateC3A contentAlkali Silica Reactivityalkali contentCorrosion of Embedded Steelcomposition (esp. C3A content)
Cement composition affects the permeability of concrete by controlling the rate of hydration. However, the ultimate porosity and permeability are unaffected by changes in rate. ACI Committee 225R, Guide to the Selection and Use of Hydraulic Cements, AC225R-85, American Concrete Institute, Detroit, Mich. 1985; Powers, T. C., L. E. Copeland, J. C. Hayes, and H. M. Mann, Permeability of Portland Cement Paste, ACI Journal Proceedings 51 (3):285-98, 1954. Coarse cement tends to produce pastes with higher porosity than that produced by fine cement. Powers et al. 1954. Cement composition has only a minor effect on freeze-thaw resistance. Corrosion of embedded steel has been related to C3A content. Verbeck, G. J., Field and Laboratory Studies of the Sulfate Resistance of Concrete, in Performance of Concrete Resistance to Sulfate and Other Environmental Conditions; Thorvaldson Symposium, 113-24, University of Toronto Press, Toronto, CA, 1968. The higher the C3A content, the more chloride that is tied into chloroaluminate complexes and unavailable for catalysis of the corrosion process.
Blended cement, as defined in ASTM C 595, is a mixture of portland cement and blast furnace slag (BFS) or a “mixture of portland cement and a pozzolan (most commonly fly ash).” The use of blended cements in concrete reduces mixing water and bleeding, improves finishability and workability, enhances sulfate resistance, inhibits the alkali-aggregate reaction, and lessens heat evolution during hydration, thus moderating the chances for thermal cracking on cooling. Blended cement types and blended ratios are given in Table 3.
TABLE 3Blended Cement Types and Blended RatiosTYPEBLENDED INGREDIENTSIP15-40% by weight of Pozzolan (fly ash)I(PM)0-15% by weight of Pozzolan (modified)P15-40% by weight of PozzolanIS25-70% by weight of blast furnace slagI(SM)0-25% by weight of blast furnace slag (modified)S70-100% by weight of blast furnace slag
Expansive cement, i.e., cement employing expansive components, is a cement containing hydraulic calcium silicates, such as those characteristic of portland cement, which, upon being mixed with water, forms a paste. During the early hydrating period after setting, expansive cement-based paste increases in volume significantly more than does portland cement-based paste. Expansive cement is used to compensate for volume decrease due to shrinkage and to induce tensile stress in reinforcement.
Expansive cement-based concrete used to minimize cracking caused by drying shrinkage in concrete slabs, pavements, and structures is termed shrinkage-compensating concrete. ACI Committee 223, Standard Practice for the Use of Shrinkage-Compensating, ACI 223-83, American Concrete Institute, Detroit, Mich., 1983.
Self-stressing concrete is another expansive cement-based concrete in which the expansion, if restrained, induces a compressive stress high enough to result in a significant residual compression in the concrete after drying shrinkage has occurred.
Three kinds of expansive cement are defined in ASTM C 845:
Type K: Contains anhydrous calcium aluminate;
Type M: Contains calcium aluminate and calcium sulfate; and
Type S: Contains tricalcium aluminate and calcium sulfate. Only Type K is used in any significant amount in the United States.
Concrete placed in an environment where it begins to dry and lose moisture will begin to shrink. The amount of drying shrinkage that occurs in concrete depends on the characteristics of the materials, mixture proportions, and placing methods. When pavements or other structural members are restrained by subgrade friction, reinforcement, or other portions of the structure, drying shrinkage will induce tensile stresses. These drying shrinkage stresses usually exceed the concrete tensile strengths, causing cracking. The advantage of using expansive cements is to induce stresses large enough to compensate for drying shrinkage stresses and minimize cracking. ACI Comm. 223 1983; Hoff, G. C. et al., Identification of Candidate Zero Maintenance Paving Materials, 2 vols, Report no. FHWA-RD-77-110 (May), U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss., 1977.
Physical and mechanical properties of shrinkage compensating concrete are similar to those of portland cement concrete (PCC). Tensile, flexural, and compressive strengths are comparable to those in PCC. Air-entraining admixtures are as effective with shrinkage-compensating concrete as with portland cement in improving freeze-thaw durability.
Some water-reducing admixtures may be incompatible with expansive cement. Type A water-reducing admixture, for example, may increase the slump loss of shrinkage-compensating concrete. Call, B. M., Slump Loss with Type “K” Shrinkage Compensating Cement, Concrete, and Admixtures, Concrete International: Design and Construction, January: 44-47, 1979. Fly ash and other pozzolans may affect expansion and may also influence strength development and other physical properties.
In Japan, admixtures containing expansive compounds are used instead of expansive cements. Tsuji and Miyake described using expansive admixtures in building chemically pre-stressed precast concrete box culverts. T suji, Y. and N. Miyake, Chemically Prestressed Precast Concrete Box Culverts, Concrete International: Design and Construction 10 (5):76-82 (May), 1988. Bending characteristics of chemically pre-stressed concrete box culverts were identical to those of reinforced concrete units of greater thickness. Tsuji and Miyake (1988).
Very-high-strength concrete (VHSC) is made from the same general constituents as conventional concrete, i.e., cementitious material, water, aggregate, and admixture for removing air and water from the mix. Careful selection of constituents and proportions, as well as proper processing, results in significant increases in both tensile and compressive strength, toughness, durability, and reduced water permeability. Physical and mechanical properties may be improved by the application of heat and pressure during casting and curing. Defining characteristics of VHSC include:                improved homogeneity through particle size and material selection;        increased density by optimization of particle size and mixing energy and technology;        improved strength by maximizing reactive materials and minimizing water content;        increased microstructure homogeneity by application of pressure before setting and post set heat treatment; and        increased tensile strength, toughness, and ductility by incorporation of reinforcing fibers (“macro” fibers), reinforcing microfibers, or both.        
Conventional concrete is very heterogeneous incorporating constituents from fine cement to coarse aggregates. Under a system of forces, each of these constituents deforms at its own rate. The differential movement of these components produces strain between the constituents that begin the process of tensile fracture when the strain exceeds the capacity of the concrete. VHSC comprises particles of similar moduli and size, contributing to a greater homogeneity of the concrete and reducing any differential tensile strain, thereby ultimately increasing the load-carrying capacity. Conventional formulations of VHSC often incorporate macro-length reinforcing fibers (macrofibers) to enhance “toughness” and have a rheology that requires retention in formwork until they are hydrated (hardened).
In selecting and mixing constituents for VHSC, particle-packing techniques are employed to maximize the solids per unit volume to achieve an optimally high “denseness,” i.e., the relative amount of volume attributed to solids. The largest particle in VHSC is the aggregate, e.g., sand, having a maximum particle size of 4.75 mm. The next largest size is that of the cement at between 10 and 100 microns (μm). The smallest size used in conventional VHSC is that of silica flume at about 0.1 μm. The higher the “denseness,” and thus, the greater the strength, the lower the permeability because the voids are fewer and smaller.
The strength of VHSC is further enhanced via incorporation of pozzolanics, such as fine siliceous or aluminous powders. These react to form hydration products. VHSC formulations employ materials with a high silica content, such as low carbon silica flume, to achieve high strength. These materials include chemically active silica that facilitates production of calcium-silicate-hydrate (C-S-H), bonding the other constituents together. Large amounts of C-S-H increase the strength of the binder, improving the bond between cement and aggregate.
To optimize a VHSC formulation, the water to cement (w/c) ratio must be controlled. For example, water needed to hydrate all of the portland cement in a mix requires a w/c ratio of about 0.4. Water not chemically or physically combined in the hydration or pozzolanic reactions weakens the resultant concrete. Thus, the volume of water in a VHSC mix is kept lower than that needed to hydrate all the cement, insuring that the water is consumed in the hydration and pozzolanic reactions. Because this low volume of water may affect workability of the mix, at least one high-range water-reducing admixture (HRWRA) may be added.
Conventional VHSC may exhibit an ultimate compressive strength near 175 MPa when processed at ambient temperature. Curing at 90° C. for a few days yields compressive strength greater than 200 MPa. Further, compression of the product during early hydration to remove excess air and water, and heating up to 400° C. on a reasonable schedule, yields VHSC having a compressive strength of 800 MPa.
The tensile strength of conventional VHSC may be made greater than conventional concrete. The tensile strength of VHSC with a compressive strength of 180 MPa may be about 10 MPa, but may be enhanced via use of steel reinforcing fibers, typically macrofibers. These fibers increase the first-crack load, the ultimate load-bearing capacity and significantly increase flexural toughness.
Conventional VHSC exhibits a near linear stress-strain relationship to failure as fabricated without reinforcing fibers, typically macrofibers, exhibiting a typical fracture energy of less than 140 J/m2. Incorporating macrofibers in a VHSC improves response in the post-first-crack region of the load-to-failure cycle. Best results have been seen using hooked-end steel macrofibers of about 0.5 mm diameter. The large number of small macrofibers crossing the path of potential cracks, coupled with the good bond between the macrofibers and the matrix, greatly increase toughness. Cargile, Dr. J. Donald et al., Very-High-Strength Concretesfor Use in Blast-and Penetration-Resistant Structures, The AMPTIAC Quarterly, Vol. 6, No. 4, 1999.
Field tests subjected thin concrete panels to fragment penetration. These panels were made from predecessor formulations to those of embodiments of the present invention. The panels experienced high resistance to dynamic loads of blast and penetration. O'Neil, E. F. et al., Tensile Properties of Very-High-Strength Concrete for Penetration Resistant Structures, Shock and Vibration, Vol. 6, Nos. 5, 6, pp. 237-245, 1999; Neeley, B. D. and D. M. Walley, VHS Concrete, The Military Engineer, Vol. 87(572), pp. 36-37, 1995; O'Neil, E. F. et al., Development of Very-High-Strength and High-Performance Concrete Materials for Improvement of Barriers Against Blast and Projectile Penetration, The 24th Army Science Conference, Presentation FO-01, Nov 29 -Dec 2, 2004, Orlando Fla. Performance in these tests indicate embodiments of the present invention are even more suitable for resisting high wind loads and flying debris (twinclude-ballistic fragments), such as may be generated by hurricanes and tornadoes. Select embodiments of the present invention envision inexpensive concrete products that provide dynamic resistance to blast and penetration forces at a level equivalent to more expensive materials, such as ceramics.